1. Field of the Invention
The present invention generally relates to a vibration absorber and, more particularly, to an active/passive distributed vibration absorber for controlling vibration and sound radiation.
2. Background Description
Active and passive noise reduction control techniques are widely known and commonly used to reduce and/or control vibrations and accompanying sound radiation in vibrating bodies, such as aircraft and the like. In many instances active noise reduction techniques adequately reduce vibrations and noise, but at the cost of expensive and complex control systems. Similarly, passive noise reduction techniques also have been known to reduce vibrations and noise, but these passive systems are typically bulky and heavy and are not effective over low vibration frequencies.
Basically, active vibration control systems use a sensor which detects vibration or noise from a vibrating body. The sensor converts the vibration or noise into a signal and then inverts and amplifies the signal. The inverted signal is then fed back to an actuator (or loudspeaker) which provides the inverted signal to the vibrating body thus reducing the vibration or noise. Active control systems are typically effective at lower frequencies such as below 1000 Hz.
In order to properly take advantage of active control systems, the selection of proper sensors and actuators are critical to the functionality of the active control systems. That is, if an improper sensor or actuator is chosen, the active control system will not properly invert and amplify the signal, and will thus not adequately reduce the vibration and noise of the vibrating body. It is also critical to the functioning of the active vibration control system to properly position the sensor and the actuator on the vibrating body with respect to one another, as well as with respect to the vibrations associated with the vibrating structure. For example, if the sensor and the actuator are not positioned properly, the inverted signal may not be properly amplified in order to cancel the vibration on the vibrating body. Also, it is very important to have a correct feedback circuit with is capable of inverting the signal since such a circuit determines the effectiveness of the vibration control and its frequency range.
In contrast to active control systems, passive damping systems usually are much less complex and costly. However, such damping systems are bulky and are generally only effective at higher frequencies of greater than 500 Hz. It is at these greater frequencies that the dimensions of the passive damping systems are comparable with the wavelength of the vibration of the vibrating body.
It is also common in the practice of vibration control systems to combine active and passive vibration systems. However, such hybrid active/passive dynamic vibration control systems provide improved attenuation over that achieved by the passive system at the expense of the energy added to the system via the control force.
Point tuned vibration absorbers are another method of damping the vibration of a vibrating body. However, a point absorber only controls one frequency at one point and is thus limited in its function to control vibrations over a large area of the vibrating body.